The influence of matric suction on the pull-out capacity of grouted soil nails

The design of soil nail systems used in engineering practice is either based on conventional soil mechanics or empirical procedures ignoring the influence of matric suction. A comprehensive experimental program was conducted to better understand the influence of matric suction on the pull-out capacity of soil nails installed in compacted sand under both saturated and unsaturated conditions.;Pull-out tests were performed on the nails installed vertically, horizontally and at an inclination of 15° to the vertical. A strong relationship was observed with the pull-out test results and the soil-water characteristic curve (SWCC) for a series of nails pulled out at 15° using a wide range of matric suction values. A technique is proposed for the estimation of the pull-out capacity of soil nails by considering the influence of matric suction, nail inclination and dilatancy. The pull-out capacity can be predicted by using the saturated soil parameters (c' and &phis;') and the SWCC in the proposed model. Pull-out test data available in the literature for both saturated and unsaturated conditions were also analyzed using the proposed technique. There is a reasonably good comparison between the measured and estimated pull-out capacity of soil nails tested in the present research program and also using the data from the literature.;The proposed technique is simple, allows for better optimization of the grout-soil adhesion and provides a better understanding of the long term performance of the pullout capacity of soil nails in different environmental conditions.;The testing program was undertaken in the laboratory using a specially designed test box with dimensions of 1.5 m x 1.2 m in plan and 1.1 m height. A single prototype soil nail was installed in the test box and tested under saturated and various average matric suction values. The size of the test box was sufficiently large to avoid influence of any boundary effects. The results of the study demonstrated that the pull-out capacity of soil nails is significantly influenced by the contribution of matric suction. The pull-out capacity of the tested nails under unsaturated conditions for the tested compacted sand ranged from 1.3 to 1.7 times higher in comparison to its capacity under saturated state.